Aluminum alloy workpieces, especially castings, are used in making many articles of manufacture. In the automobile industry, for example, many engine and transmission parts, chassis parts, body parts, and interior parts are made of silicon-containing aluminum alloy castings. Many of these parts such as engine blocks, cylinder heads, crank cases, transmission cases, and the like are initially formed as castings using sand molding, permanent mold, high pressure die casting, and lost foam processes. Following these casting process, many surfaces of the parts still have to be machined to specified dimensions within very close tolerances.
Aluminum alloy 319 (“AA319”) is an example of an aluminum alloy that is used to form cast shapes of many of the above parts. A representative nominal composition, by weight, of AA319 is 5.5 to 6.5% silicon, up to about 1% iron, 3 to 4% copper, about 0.5% manganese, about 0.1% magnesium, about 0.35% nickel, 1-3% zinc, about 0.25% titanium, and the balance substantially aluminum. The silicon content contributes fluidity to the molten alloy for the pouring of intricate cast shapes. But the silicon, and sometimes other alloying elements, form secondary phases in the predominately aluminum matrix that tend to abrade machining tools.
Engine and transmission castings, for example, may require precision machining processes such as milling, drilling, honing, and reaming. In these machining processes the casting is carefully positioned in a fixture and a cutting tool, carried and powered by an operator or computer controlled machine tool, cuts a cast surface to remove chips of workpiece metal to bring the surface to a specified finish quality and dimension. The tools for metal removal may be made of a hard material, especially the surface of the tool that engages the workpiece. For example, tools may be made of a tool steel composition or of a sintered composition comprising tungsten carbide, or the like. Sometimes the cutting surface of the tool may be coated with a hard carbon material such as polycrystalline diamond or diamond-like carbon.
The tendency of Al and its alloys to adhere to tool surfaces may create enormous difficulties in metal forming operations such as rolling, forging, casting, extrusion, and machining. Aluminum has a 3 to 6 nm thick natural oxide layer, which is locally destroyed during the forming process, exposing nascent Al metal that is highly chemically reactive. Once the oxide layer is destroyed, the onset of gross adhesive transfer to the tool surface is likely: this introduces process instability and threatens the success of the deformation process.
In conventional metal removal operations, the machined surface may be flooded with a machining liquid (water based or oil based) for the purposes of cooling and lubricating the region impacted by the cutting tool. The lubrication promotes cutting by minimizing adherence between the tool and the machined surface. Ultimately, the machining liquid must be drained from the machining area for recovery processing and re-use, or for disposal.
Wear resistance of a tool for cutting or otherwise machining a workpiece may be improved by coating a contact area of the tool with a hard material layer. Suitable hard coating materials for these tools often include carbon, common examples of which are diamond, hard carbon, and various forms of mixtures of sp2 and sp3 hybridized amorphous carbon.
Wet or dry machining processes may be used to machine a workpiece using the hard coated tools. Drawbacks with using wet machining processes, for example, include relatively high costs of performing the wet machining process and, in some cases, environmental concerns potentially linked to use of coolants or other machining fluids. Dry machining processes, especially for machining soft metals such as aluminum alloys, for example, may lead to adhesion of the aluminum alloy workpiece to the tool surface, thereby diminishing the operative life of the tool.
It is an object of this invention to provide a method for machining aluminum alloy workpieces (e.g., castings) without the use of such a machining liquid. In accordance with this invention such a practice is termed “dry machining.”